1. Field of the Invention
The present invention relates to radar and ground penetrating radar imaging methods, and more particularly to radars that linearly sweep through frequencies and synchronously detect return signals affected by time-of-flight delays to target objects.
2. Description of Related Art
Frequency modulated (FM) continuous wave (CW) radars transmit a sweep frequency that is mixed with a return echo to produce a beat-frequency. The beat frequency output from the mixer is a function of both how fast the CW output is sweeping in frequency and how far the return echo had to travel from the transmitter to the target and back to the receiver. A faster FM sweep of the CW signal increases the scale of the beat frequency product. Given a linear sweep, e.g., a sawtooth, and a fixed distance to the radar target, then the beat frequency will be a steady tone. The bandwidth of that tone determines the resolution of the radar.
The beat frequency tone represents the range of the target in the frequency domain. Traditional radars launch radio frequency (RF) pulses that are delayed in their echo returns in time by how far they had to fly out and back. So traditional radars produce range signals in the time domain. A fast Fourier transform (FFT) is typically used to convert FM-CW radar frequency-domain range signals to time-domain range signals so they can be conventionally processed.
Yukinori Yamada describes an FM-CW radar in U.S. Pat. No. 6,121,917, issued Sep. 19, 2000. An array antenna, beat signals, and Fourier transform process are used to Fourier transform data from each beam angle. Such radar determines the range to an object near the antenna array.
Yukinori Yamada describes another FM-CW radar apparatus in U.S. Pat. No. 6,445,339 B1, issued Sep. 3, 2002. The transmitted signal used is a frequency modulated continuous wave. A beat signal is generated from mixing transmitted and received signals, and this is the A/D converted. A switch is used to select various antenna elements in an array. A digital signal processor executes a digital beam-forming operation to detect the target from the beat signals.
The phase delays imparted by electronic FFT devices are not constant and vary with frequency. In order to get a accurate conversion between the frequency and time domains, the FFT output needs to be appropriately phase compensated. But to do this, the frequency of the signal being processed must be known to apply the appropriate correction. In FM-radar, the frequency of the return echo signal is unpredictable because it depends on the unknown range to the target. Prior art has neither recognized this source of error nor have there been any solutions proposed in conventional radar implementations.